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WASHINGTON - Massachusetts almost certainly will lose one of its 10 congressional districts after next year’s census, the result of a long-term population shift that is giving Southern and Western states more political power in Washington at the expense of the Northeast, say specialists who have been poring over data in advance of the 2010 count.

Long-term economic and demographic shifts in favor of warmer climates with less expensive housing are to blame for the state’s slower growth, and thus the loss of a congressional district, according to estimates. Massachusetts’ population grew by just 2.3 percent from 2000 to 2008, compared with 8 percent nationally, a disparity that is expected to continue next year and beyond.

“We did five different scenarios projecting the population forward, and in each of those five scenarios, Massachusetts would lose a seat,’’ said Kimball Brace of Election Data Services, a political consulting firm in Washington. Demographic trends are “not a good sign from a Massachusetts standpoint. You’ve got a long way to go to keep the seat.’’

Several other Northeastern and Midwestern states, including New York and New Jersey, are also expected to lose seats, said Brace.

For the Bay State, the expected delegation downsizing would occur at an especially inopportune time, just as the state has reached its highest degree of clout in the US House of Representatives in two decades. The all-Democrat group includes members such as Edward Markey and Barney Frank, who lead powerful committees and are shaping historic legislation, as well as other members who wield extensive influence on powerful budget-setting and taxation committees.

The census changes would require a redistricting before their reelection races in 2012. Several members of the delegation said that they are prepared to deal with a potential cut, but that they do not know which among them would be forced out. Redistricting would be up to the Legislature.

“I haven’t got any control over it, so why worry about it?’’ said Representative Michael E. Capuano, Democrat of Somerville. “I don’t think there’s anyone around who has figured out how to stop the population flow to the Southwest.’’

The last time the state lost a seat, after the 1990 Census, it set off intense infighting among the state’s Democrats, confrontations with then-governor William F. Weld, and even an ultimately futile legal challenge that went all the way to the US Supreme Court.

If anything, the state has more to lose now, because so many of its representatives have risen to senior leadership posts at a time of Democratic dominance in Washington.

“Everybody in the delegation is particularly well positioned with their committee assignments,’’ said Representative Richard E. Neal, Democrat of Springfield, a member and subcommittee chairman on the Ways and Means Committee. “It obviously would present a challenge for the state.’’

Losing a seat would also reduce the state’s votes in the Electoral College from 12 to 11.

In interviews, members of the delegation said the possibility of losing a seat was troubling for the state, but adopted a philosophical stance when asked about the potential impact on their careers.

“I’ve got plenty of other things to worry about right now,’’ said Representative Stephen F. Lynch, Democrat of South Boston.

In the meantime, the state is spending $2 million on census outreach, targeting hard-to-reach immigrant and minority communities where distrust of government has historically been higher. Markey, the senior member of the delegation, said he rejected the idea that the state would inevitably lose a seat.

“I believe that if we count every person in the state that we still have a chance to hold on to the seat,’’ he said.

William F. Galvin, the secretary of state and the lead Massachusetts official on the census, also said predictions of losing a seat were “speculative.’’ He pointed out that similar predictions were made before the 2000 Census, which ultimately maintained the status quo.

Increasing participation in the census is crucial, Galvin said. Not only does it determine representation in Congress, but it is also used to set federal funding for dozens of health care, transportation, and education programs.

Paradoxically, the main factors that could play in the state’s favor are the weak national economy and the foreclosure crisis, because they might check the rapid growth in the Sun Belt, he said.

The most likely outcome of redistricting would be to place two incumbents in the same district, forcing them to run against each other. In many states a nonpartisan board handles redistricting, but in Massachusetts the Legislature draws the borders and has a long history of gerrymandering. Massachusetts’ House speaker, Robert A. DeLeo, declined to comment.

Until 1920, most states added representatives every 10 years, until Congress capped its own membership at 435 amid fears that the body was growing so large it would lose its collegiality. Since then, the number of representatives from New England has dwindled from 32 to 22.

James Brett, a former Massachusetts state legislator who chaired the state’s redistricting committee in 1990 and now runs the New England Council, said the six states had to face up to the reality.

“It’s not a good sign when we lose a voice going to Washington,’’ he said. “That’s why it’s important the delegation work as a region - because we tend to be losing these seats every time there’s a decennial.’’